I had a broken Tima KK402 calculator which is now only a few bucks. Therefore, it is worth the risk of disassembling since there is almost nothing to lose as I can always buy a new calculator. In fact, the experience of disassembling it is worth it. Anyone can know if the problem is the cable is cut. Although I have an advantage of able to identify disconnected cable using multimeter even if it seemed to be intact and know the optimal way to connect or replace them since I had a background in electrical engineering.
Many beginners would probably disassemble every detail part but there is a step before that that is risk free of making it permanently broken. That is to just clean and give it a try whether it is fixed or not. Dirt and dust can contribute to the malfunction of electric circuits. Unfortunately for me, this step is not enough.
Then I checked the printed circuit board (PCB) and found that it was not aligned well with the buttons. After examining the surroundings, some of the holders broke which cause the misalignment. The solution is to align it and find alternative parts to function as the holder.
Finally it worked. Almost no risk as the calculator only worth a few bucks. Instead, know what is inside the calculator and gained some experience. Make a video recording like me just in case we forget of how to reassemble after disassembling.